The present invention relates to a centrifugal extractor of non-contact journaled construction using magnetic bearings, and in more detail, relates to a centrifugal extractor of non-contact journaled construction devised, in extracting and separating U (uranium) and Pu (plutonium) under the environment of corrosive gas and mist-like liquid in a reprocessing process of a spent nuclear fuel, so as to be able to plan the high reliance and long service life.
There is a Purex (Plutonium Reduction Oxidation) process which is one of reprocessing methods for a spent nuclear fuel produced from nuclear facilities. This process is, as is well-known, a process for dissolving a spent nuclear fuel into nitric acid, and separating and recovering U and Pu dissolved in the nitric acid aqueous solution by solvent extraction operation.
There are some kinds of apparatuses for carrying out the solvent extraction operation, one of which is a centrifugal extractor for carrying out separation of two liquid phases (an aqueous phase and an organic phase) by centrifugal force. This centrifugal extractor is designed so that a rotor is supported rotatably in a state of being suspended by a main shaft in a housing, an aqueous phase and an organic phase are supplied to the outer circumference of the rotor and mixed between the housing and the rotor, the mixed phase is sucked into the rotor and separated into two phases in a centrifugal force field internally of the rotor, and the separated phases are discharged to corresponding collectors.
Such a centrifugal extractor as described above has advantages that processing speed is high, that since the rising time of apparatus is quick, an operating rate is enhanced and equipment can be simplified, that apparatus can be designed to be a smaller type, that deterioration of radiation of a solvent can be reduced, and the like, as compared with other extractors (for example, such as a mixer-settler extraction apparatus, a pulse column extraction apparatus, etc.). Therefore, this centrifugal extractor is said to be advantageous for reprocessing a fast breeder fuel of high burnup and high Pu-enrichment, and study and development thereof have been progressed.
Incidentally, in the re-processing process for a spent nuclear fuel, since nuclear fuel materials being dissolved in nitric acid is solvent-extracted using TBP (tributyl phosphate) as an extracting solvent, nitric acid mists are generated. Accordingly, it is necessary for the centrifugal extractor to have materials and construction capable of resisting use under the environment of corrosive gas and mist-like liquid as described above.
The centrifugal extractor for the reprocessing for a spent nuclear fuel developed so far is of a spindle construction in which a rolling bearing is used to support the main shaft for suspending the rotor. From the result of the service life test, stainless steel (SUS 440C) is used as materials for inner ring and outer ring of the rolling bearing, a rolling body, a holding unit or the like.
However, the interior of the centrifugal extractor used in the reprocessing process for a spent nuclear fuel is exposed to the nitric acid mists generated on the process, as described above, and therefore, rust in the rolling bearing and deterioration of a lubricating agent (grease) occur, and operation for a long period of time cannot be carried out without maintenance unless the measures against the nitric acid mists such as air purge are taken. Further, even if ceramic materials are sued in place of the stainless steel (SUS 440C), the lubricating agent unavoidably deteriorates, making it difficult to operate continuously for a long period of time.
As is known, there is a magnetic bearing as one kind of a bearing used in rotating machines. This magnetic bearing is a bearing of construction in which the magnetic force is exerted to levitate a shaft whereby a rotating element is supported with respect to a fixed element in a non-contact manner, which magnetic bearing is applied to a rotating machine for which high speed rotation (for example, 10000 rpm or more) is exclusively required. In the case of the centrifugal extractor used in the reprocessing process for a spent nuclear fuel, the magnetic bearing is not used for the reasons such that the rotational speed is relatively low (for example, approximately 3000 to 4000 rpm), that since an electromagnet (coil) is required for generation of a magnetic field, there poses a problem in corrosion/deterioration caused by the nitric acid mists or the like, and therefore, the rolling bearing is used at present as mentioned previously.